Nonwoven fabrics (cloths) comprising a natural fiber or a synthetic fiber have been widely used not only for hygiene or medical applications (such as a disposal diaper or a wet wiper) and clothing applications, but also for industrial applications. The nonwoven fabrics are thus important to wide-ranging applications including a common material for living, an industrial material, and the like. In particular, a highly soft nonwoven fabric (usually such as a needle-punched nonwoven fabric or a hot-airthrough-nonwoven fabric) is in widespread use as a bulky and light nonwoven fabric. In order to impart hardness to such a soft nonwoven fabric, it is necessary to process the soft nonwoven fabric by a treatment such as a heat-press treatment or a resin impregnation.
However, in a heat-pressed nonwoven fabric only the fibers close to a surface of the nonwoven fabric are bonded to each other (or together), but the fibers inside the nonwoven fabric are not enough bonded to each other. It is thus difficult to produce a nonwoven fabric having an enough hardness by the heat-press treatment. Since it is necessary that the inner fibers be also melt-bonded together firmly to impart an enough hardness to the nonwoven fabric, in the heat-press treatment, the nonwoven fabric has to be subjected to an excessive heating due to its slow heat transfer to the inner fibers. However, the excessively heated nonwoven fabric has surfaces in which the fibers are more strongly or firmly bonded together to form high-density layers. After all, even with the excessive heating it is difficult to impart a sufficient hardness to the nonwoven fabric. Furthermore, in a nonwoven fabric impregnated with a resin for imparting hardness thereto, the voids between the fibers in the nonwoven fabric are filled up with the resin, which consequently render the nonwoven fabric highly dense.
In addition, Japanese Patent Application Laid-Open No. 314592/2004 (JP-2004-314592A, Patent Document 1) discloses a fiber aggregate board comprising kenaf fibers, which is obtained by fibrillating a kenaf, bonded together with a thermosetting adhesive agent as a hard nonwoven fabric board comprising a natural fiber. The fiber board has a density of 600 to 900 kg/m3. This fiber board is generally referred to as “kenaf board”. Although the kenaf, a raw material for the kenaf board, is a natural fiber, the kenaf fiber is impregnated with an adhesive agent and subjected to a press to form a board material at a board forming step. Such a kenaf board is used as an alternative to a wood or a timber for a building material (e.g., a roof cover and a flooring material), furniture (e.g., a storage case, a built-in kitchen, and a closet), an electrical equipment (e.g., a speaker), a musical instrument (e.g., a piano and an organ), or a table-tennis table.
However, the use of a phenolicresin-series adhesive agent or the like is inevitable for producing the board having an enough hardness or strength from the kenaf as a raw material. Thus there arises a concern about a danger to public health due to a formaldehyde emission or generation from the board. Moreover, the kenaf board was developed as an alternative to a wood or a timber as mentioned above and has no air-permeability or a very low air-permeability.
Furthermore, boards used for applications [for example, a filter for an automobile or a machine, a fan filter, a building material, or a furniture (such as a built-in kitchen)] require flame retardancy besides hardness. A flame-retardant board is commonly known as such a board. The flame retardancy thereof is attained by impregnating glass fibers with a flame-retardant resin or by adding a flame retardant containing a halogenated compound or an antimony compound to a board in a post-processing. For example, Japanese Patent Application Laid-Open No. 221453/2003 (JP-2003-221453A, Patent Document 2) discloses a polyester fiber board having rigidity and flame retardancy as a hard and flame-retardant board comprising a synthetic fiber. The polyester fiber board is obtained by forming a composite coating comprising an organic binder and an inorganic powder on a surface of a polyester fiber or by filling a composite material comprising an organic binder and an inorganic powder into the pores of a board comprising a polyester fiber. This document teaches that slurry comprising an inorganic powder and an organic binder is injected by pressure into a nonwoven fabric comprising a polyester fiber to impart rigidity and flame retardancy to the board.
However, the complex step of the process for the slurry injection into the nonwoven fabric and the time-consuming slurry injection prevent the quality assurance and the increase of the processing speed. Moreover, in the process, the voids between the fibers constituting the nonwoven fabric are filled up with the inorganic powder or the binder, whereby the density and weight are increased.
On the one hand, a wood fiberboard (e.g., a particle board and an MDF: Medium Density Fiber Board) is known as a board material having a lightness in weight and a high bending strength, which is made of wood chips as a main raw material and an adhesive agent and formed by virtue of heat and pressure [see Japanese Patent Application Laid-Open No. 31708/1994 (JP-6-31708A, Patent Document 3), Japanese Patent Application Laid-Open No. 155662/1994 (JP-6-155662A, Patent Document 4), and Japanese Patent Application Laid-Open No. 116854/2006 (JP-2006-116854A, Patent Document 5)].
However, the wood fiber board is usually heavy and imposes physical strains on workers installing the board. Additionally, during bending the wood fiber board by applying a high impact or a load thereon, the board is suddenly broken and easily damaged. Moreover, the wood fiber board reuses a wood waste with an intention for preserving resources. The wood fiber board is developed for the above-mentioned applications as an alternative to a wood or a timber and usually has no air-permeability as well as the kenaf board. Furthermore, the wood fiber board often contains a melamine resin as an adhesive agent, whereby formaldehyde is emitted from the board.
On the other hand, Japanese Patent Application Laid-Open No. 235558/1988 (JP-63-235558A, Patent Document 6) discloses a nonwoven fabric comprising an ethylene-vinyl alcohol copolymer fiber having a predetermined mole ratio of ethylene as a nonwoven fabric comprising a thermal (heat) adhesive fiber under wet. An object in this document is to obtain a nonwoven fabric which is bulky, soft, and strong enough. To achieve the above-mentioned object, the ethylene-vinyl alcohol copolymer is firmly bonded together by allowing the copolymer to swell in water and heating the swollen copolymer in contact with a heater (or a heating element). That is, the obtained nonwoven fabric is soft, not hard.
Moreover, Japanese Patent Application Laid-Open No. 123368/2001 (JP-2001-123368A, Patent Document 7) discloses a self-forming porous fiber aggregate containing fiber webs bonded together firmly as a light-weight and bulky fiber aggregate nonwoven structure. The self-forming porous fiber aggregate is obtained by heating the fiber web to bond an ethylene-vinyl alcohol copolymer fiber to fibers constituting the fiber aggregate by a wet and heat treatment. In this document, the above-mentioned fiber aggregate having cell-like voids formed therein is produced by immersing a fiber aggregate nonwoven structure comprising a thermal (heat) adhesive fiber under wet in water having a room temperature, subjecting the fiber aggregate nonwoven structure containing the water to a wet-heat treatment in which the fiber aggregate nonwoven structure is heated at about 100° C. to generate air bubble therein, and cooling the resulting fiber aggregate nonwoven structure.
Owing to the internally formed cell-like voids, the fiber aggregate nonwoven structure is bulky and light. However, the fiber aggregate nonwoven structure easily deforms or breaks at a part or area having such voids. It is still difficult to provide the fiber aggregate nonwoven structure having a high hardness.                [Patent Document 1] JP-2004-314592A        [Patent Document 2] JP-2003-221453A        [Patent Document 3] JP-6-31708A        [Patent Document 4] JP-6-155662A        [Patent Document 5] JP-2006-116854A        [Patent Document 6] JP-63-235558A        [Patent Document 7] JP-2001-123368A        